Heavy duty liquid detergent compositions are generally comprised of a solvent and one or more of the following: an anionic detergent component, alkali metal hydroxide, a nonionic detergent component, fatty acid, a hydrotrope, a dye, a builder and a buffering agent such as an alkanolamine or sodium silicate. Optonally, a perfume and opacifier may be incorporated.
The mixing and order of addition of these components has conventionally been as follows. Most of the solvent is introduced into a vessel. Each of the following components is then added in the order listed with moderate stirring; as soon as the addition of one component is complete, addition of the next begins.
Dye PA2 Hydrotrope PA2 Alkali Metal Hydroxide PA2 Anionic Detergent (acid form) PA2 Nonionic Detergent, Fatty Acid PA2 Builder PA2 Buffering Agent PA2 Opacifier PA2 Perfume PA2 Solvent (remaining)
The contents of the vessel after addition of any of these ingredients to the water is referred to herein as the "vessel mixture".
Sufficient alkali metal hydroxide is added to the vessel mixture to convert the anionic detergent (acid form) to the salt form and the fatty acid to soap when these components are added in their separate addition steps. "Neutralization" is the name in this specification for the process step in which the anionic detergent component is converted from the acid to the salt form. (Soap formation is not considered part of "neutralization", mainly because much more detergent than fatty acid is neutralized.)
Neutralization may be accomplished by adding the alkali metal hydroxide to a vessel mixture already containing the anionic detergent (acid form) of the batch. However, the unneutralized acid form may corrode the vessel and its mixing means. Also, addition of the hydroxide to the detergent in acid form may produce gummy solids which are difficult to dissolve. Both these problems are avoided by instead adding the anionic detergent (acid form) component to the vessel mixture already containing the alkali metal hydroxide.
Neutralization is exothermic. When the detergent composition is made in large batches, e.g. of 100,000 lbs. and more, the potential for heat release is very large. The heat release accompanying neutralization raises the temperature of the entire aqueous mixture in the mixing vessel. Care must be taken not to exceed the temperature limits of the vessel mixture of any of its 25 components. A temperature maximum of 130.degree.-150.degree. F. or 135.degree.-145.degree. F. applies to the process. The temperature of the mixture is maintained below the maximum by a controlled rate of addition of the anionic detergent in acid form to the aqueous mixture, preferably with interruptions in the addition to allow the mixture to cool.
It has been noted that the above liquid detergent compositions do not always retain their color. In fact, the process of mixing the composition components often seems to affect the dyes adversely: the blue and/or violet color of these compositions may be pale at the end of component mixing and even before storage may be a muddy steel grey. This color degradation may continue and worsen during storage, but this degradation of color is apparent in those batches showing it immediately upon completion of mixing the batch. Adding the dye to the vessel mixture after (instead of before) the neutralization step does not prevent color degradation.
One successful response to the degradation of color has been to add extra colorant to the composition after neutralization. This solution however is unattractive because it adds expense to the process and lengthens it: extra time is needed to add and dissolve the extra dye and to perform a color matching process between each batch and a standard color sample.